Renin is an endopeptidase (molecular weight about 40,000) produced and secreted by the juxtaglomerular cells of the kidney. Renin has a high specificity for and cleaves the naturally-occurring plasma glycoprotein, angiotensinogen, at only the 10, 11 peptide bond, i.e., between the 10th (Leu) and 11th (Leu) amino acid residues in the equine substrate, as described by Skeggs et al, J. Exper. 26ed. 1957, 106, 439, or between Leu 10 and Val 11 in the human renin substrate, as elucidated by Tewksbury et al., Circulation 59, 60, Supp. II: 132, October 1979.
This cleavage of its tetradecapeptide substrate, angiotensinogen, splits off the hemodynamically-inactive decapeptide, angiotensin I, which is converted in the lungs, kidney or other tissue by angiotensin-converting enzyme (ACE) to the potent pressor octapeptide, angiotensin II. Angiotensin II then causes constriction of the arterioles and is also believed to stimulate release of th sodium-retaining hormone, aldosterone, from the adrenal gland, thereby causing a rise in extracellular fluid volume. Thus, the renin-angiotensin system plays an important role in normal cardiovascular homeostasis and in some forms of elevated blood pressure (hypertension).
Inhibitors of angiotensin I converting enzyme have proven useful in the modulation of the renin-angiotensin system. Consequently, specific inhibitors of the catalytic and rate-limiting enzymatic step that ultimately regulates angiotensin II production, the action of renin on its substrate, have also been sought as effective investigative tools, as well as therapeutic agents in the treatment of hypertension and congestive heart failure.
Renin antibody, pepstatin (another aspartic proteinase, like renin), phospholipids, and substrate analogs, including tetrapeptides and octa- to tridecapeptides, with inhibition constants (K.sub.i) in the 10.sup.-3 to 10.sup.-6 M region, have been studied.
Umezawa et al., in J. Antibiot. (Tokyo) 23: 259-262, 1970, reported the isolation of a peptide, pepstatin, from actinomyces that was an inhibitor of aspartyl proteases such as pepsin, cathepsin D, and renin. Gross et al., Science 175: 656, 1972, reported that pepstatin reduces blood pressure in vivo after the injection of hog renin into nephrectomized rats. However, pepstatin has not found very wide application as an experimental agent because of its limited solubility and its inhibition of a variety of other acid proteases in addition to renin.
Many efforts have been made to prepare a specific renin inhibitor based on pig renin substrate analogy, since such analogy has been shown to correlate well with and predict human renin inhibitor activity. The octapeptide amino acid sequence extending from histidine-6 through tyrosine-13 ##STR2## has been shown to have kinetic parameters essentially the same as those of the full tetradecapeptide renin substrate.
Kokubu et al., Biochem. Pharmacol., 22, 3217-3223, 1973, synthesized a number of analogs of the tetrapeptide found between residues 10 to 13, but while inhibition could be shown, inhibitory constants were only of the order of 10.sup.-3 M. Analogs of a larger segment of renin substrate have been also synthesized, e.g., Burton et al., Biochemistry 14: 3892-3898, 1975, and Poulsen et al., Biochemistry 12: 3877-3882, 1973, but a lack of solubility and weak binding (large inhibitory constant) generally resulted.
Modifications to increase solubility soon established that the inhibitory properties of the peptides are markedly dependent on the hydrophobicity of various amino acid residues. These modifications also established that increasing solubility by replacing lipophilic amino acids with hydrophilic isosteric residues can become counter-productive. Other approaches to increasing solubility have also had limited success.
Modifications designed to increase binding to renin have also been made, but here too, with mixed results.
A series of inhibitors of renin have been disclosed which contain the unnatural amino acid, statine: see, e.g., Veber et al, U.S. Pat. Nos. 4,384,994 and 4,478,826; Evans et al, U.S. Pat. No. 4,397,786; Boger et al, Nature, 1983, 303, 81-84 and U.S. Pat. Nos. 4,470,971; 4,485,099; 4,663,310 and 4,668,770; Matsueda et al, EP-A 128 762, 152 255; Morisawa et al., EP-A 186 977; Riniker et al, EP-A 111 266; Bindra et al, EP-A 155 809; Stein et al, Fed. Proc. 1986, 45, 869; and Holzemann et al, German Offenlegungsschrift DE No. 3438545. Attempting to explain the effect of statine, by Powers et al., in Acid Proteases, Structure, Function and Biology, Plenum Press, 1977, 141-157, observed that in pepstatin, statine occupies the space of the two amino acids on either side of the cleavage site of a pepsin substrate and by Tang et al., in Trends in Biochem. Sci., 1: 205-208 (1976) and J. Biol. Chem., 251: 7088- 94, 1976, pointed out that the statine residue of pepstatin resembles the transition state for pepsin hydrolysis of peptide bonds.
Renin inhibitors containing other peptide bond isosteres, including a reduced carbonyl isostere have been disclosed by M. Szelke et al, in work described in published European patent application Nos. 45 665 and 104 041; in U.S. Pat. No. 4,424,207, and in PCT Int. Appl. WO 84/3044; in Nature, 299, 555 (1982); Hypertension, 4, Supp. 2, 59, 1981; and British Pat. No. 1,587,809. In Peptides, Structure and Function: Proceedings of the Eighth American Peptide Symposium, ed. V. J. Hruby and D. H. Rich, p. 579, Pierce Chemical Co., Rockford, IL, 1983, Szelke et al also showed isosteric substitutions at the Leu-Leu site of cleavage, resulting in compounds with excellent potency.
Other peptide bond isosteres have then been disclosed in Buhlmayer et al in EP-A 144 290 and 184 550; Hester et al, EP-A 173 481; Raddatz, EP-A 161 588; Dann et al, Biochem. Biophys, Res. Commun. 1986, 134, 71-77; Fuhrer et al, EP-A 143 746; Kamijo et al, EP-A 181 110; Thaisrivongs et al, J. Med. Chem., 1985, 28, 1553-1555; Ryono et al., EP-A 181 071; and Evans et al, U.S. Pat. No. 4,609,641.
Other modifications which have been tried include preparing renin inhibitors with non-peptide C-termini, such as disclosed in European Published Applications 172 346 and 172 347; Evans et al, J. Med. Chem., 1985, 28, 1755-1756; and Bock et al, Peptides, Structure and Function: Proceedings of the Ninth American Peptide Symposium, ed. C. M. Deber et al, pp. 751-754, Pierce Chemical Co., Rockford, IL, 1985. Kokubu et al, in Hypertension, 1985, 7, Suppl. I, p. 8-10 and Matsueda et al, in Chemistry Letters, 1985, 1041-1044 and in European Published Applications 128 762 and 152 255 disclosed peptide aldehyde renin inhibitors, and Hanson et al in Biochem. Biophys. Res. Commun. 1985, 132, 155-161, reported peptide glycol inhibitors.
These various renin inhibitors all generally comprise peptide-based inhibitors in which a sequence of the type: . . . A-B-D-E-F-G-J-K-L . . . , where G is a peptide bond mimic and A,B,D,E,F,J,K, and L may individually be absent or may represent naturally-occuring or modified amino acids. Typical sequences of this type include: ##STR3## or ##STR4## where the N-terminus typically comprises an amino acid protecting group such as BOC or CBZ, and the N-terminal amino acids are Pro-Phe-His or Phe-His.
Replacements for the Phe(8)-His(9) portion of these sequences have been described among these references, wherein other aromatic amino acids (Tyr, Trp, etc) or substituted aromatic amino acids are generally mentioned as replacements for Phe. Then, other amino acids (e.g., Lys, Leu) have been suggested as replacements for His, or amino acids of the form, NH.sub.2 --CH(CH.sub.2 Het)CO.sub.2 H, wherein "Het" is a mono- or bicyclic heterocycle, have been substituted for either His or Phe. These 8-9-substituted sequences include peptides in which various peptide bonds have been N-methylated or reduced in order to stabilize the resulting inhibitor against enzymatic degradation. Otherwise, however, no advantage in renin-inhibitory potency or in pharmacological properties has been demonstrated or suggested by making such substitutions for the Phe-His sequence.